All you need to do is click inside the text frame and you will see a blinking insertion point cursor. You can:
start typing once you see the insertion point, or
use Edit > Paste to copy the contents of the clipboard, or
use File > Import > File to import a Word document at that location, which gives you the opportunity to map Word styles to FrameMaker tags.
If you see an arrow cursor in place of the insertion point, double click.
A super-common beginner mistake is for a new user to add pages when they want to add text. In most cases, you don't need to do that. When you start typing (or pasting or importing) FrameMaker will add new pages to the document as necessary because the primary text frame (aka Flow A) has an auto-connect property turned on, as a default.